A. Field
The present invention relates to a regulating device having a balance and a plane hairspring for a time piece movement.
B. Related Art
It is known that the turns of a plane hairspring deform eccentrically when the hairspring is in operation. This eccentric deformation of the turns, which is explained by the fact that the center of gravity of the hairspring does not correspond to the center of rotation of the balance and hairspring assembly, disturbs the setting of the balance and hairspring assembly and makes it anisochronous.
The center of gravity of the hairspring could be returned arbitrarily to the center of rotation of the balance by being shifted, however that would not solve the problem since during the working of the hairspring the center of gravity would move and would therefore no longer coincide with the initial center of gravity.
Two different solutions have been proposed for causing the center of gravity and the center of rotation to coincide while a plane hairspring is working, thereby making the deformations of the turns concentric:
the Breguet hairspring with a so-called Philips curve in which an outer curve is moved into a second plane above the hairspring plane; and
the hairspring with angle strip as described in 1958 by Messrs. Emile and Gaston Michel in an article entitled “Spiraux plats concentriques sans courbes” [Concentric flat hairsprings without curves], published by Société Suisse de Chronométrie.
The first solution amounts to modifying the initial plane hairspring so that it becomes a hairspring occupying a plurality of planes. That solution does not come within the ambit of the present invention which relates only to plane hairsprings.
The second solution consists in stiffening a determined portion of a turn by giving it the shape of an angle strip. The angle strip is situated either in the outer turn or in a central turn. Nevertheless, according to the authors of that solution, although angle strip in the central turn does indeed provide a significant improvement in terms of the isochronism of the balance and hairspring assembly, angle strip in the outer turn does not give satisfaction. Said authors even abandoned the hairspring with angle strip in the outer turn, in the belief that they had been wasting their time on this topic.